1. Field of the Invention
This invention relates to an information signal recording and reproducing apparatus and more particularly to an apparatus of the kind arranged to record in rotation pilot signals of a plurality of kinds having different frequencies, in respective different recording tracks, along with an information signal on a recording medium and to reproduce the information signal from the recording medium.
2. Description of the Related Art
The conventional recording and reproducing apparatuses of the above-stated kind include, for example, a video tape recorder (hereinafter referred to as VTR) of a helical scanning type which is arranged to perform tracking control during reproduction by the so-called four-frequency method. The manner in which tracking control signals have been obtained in accordance with the four-frequency method is as briefly described below with reference to the accompanying drawings. FIG. 1 shows magnetic recording tracks formed by the VTR of the type performing tracking control by the four-frequency method. FIG. 2 shows in a block diagram the essential circuit arrangement required for obtaining a tracking error signal. In FIG. 1, an arrow 2 indicates the direction in which a magnetic tape 1 is to be moved. Recording tracks A1, B1, A2, B2, --- are formed during recording by heads A and B which have different azimuth angles. An arrow 3 indicates the scanning direction of these heads. In the recording tracks which as a whole denoted by a reference numeral 4 are recorded, along with a video signal, pilot signals of four different frequencies f1 to f4. One of the pilot signals is recorded for every field of the video signal, that is, in each of the tracks in rotation. The sequence in which the pilot signals are recorded is as shown in FIG. 1. For example, the pilot signal of frequency f1 which is 102.5 KHz=6.5 fH (fH: the frequency of a horizontal synchronizing signal) is recorded in the recording track A1; the pilot signal of frequency f2 which is 118.9 KHz=7.5 fH is recorded in the track B1; the pilot signal of frequency f3 which is 165.2 KHz=10.5 fH in the track A2; and the pilot signal of frequency f4 which is 148.7 KHz=9.5 fH in the track B2. These pilot signals are recorded in a state of being superimposed on a video signal. The frequency difference between the pilot signals recorded in adjacent recording tracks is either fH or 3 fH as shown in FIG. 1. When the head is scanning the tracks Ai (i: 1, 2, 3, ---), the frequency difference is always fH between the pilot signal of the track being mainly scanned and that of the adjoining track on the right-hand side and always 3 fH between the pilot signal of the mainly scanned track and that of another track adjoining on the left-hand side. Further, when the head is scanning the tracks Bi (i: 1, 2, 3,---), the frequency difference is always 3 fH between the pilot signal of the mainly scanned track and that of the track adjoining on the right-hand side and always fH between the pilot signal of the mainly scanned and that of another adjoining track on the left-hand side.
Since the frequencies f1 to f4 of the pilot signals are relatively low, the pilot signals recorded in the adjacent tracks other than the mainly scanned track can be reproduced by the head as cross-talks even in the event of azimuth recording. Assuming that the head is mainly scanning the track A2, the pilot signal thus detected is a composite signal including components of frequencies f4, f2 and f3. In case that the center of the tracing locus of the head precisely coincides with the center line of the track which is mainly scanned under the tracking control, i.e. in the case of an on-track state, the pilot signals (of frequencies) f2 and f4 of the neighboring tracks are reproduced at even levels. However, the level of the frequency component f4 becomes higher than that of the other frequency component f2 when the position of the head slightly deviates from the center line of the track A2 toward the track B2 and lower than that of the component f2 when the position of the head deviates toward the track B1.
Therefore, the deviating direction and the deviating degree of the head from the mainly scanned track are obtainable by separately taking out the difference signals which represent the frequency differences fH and 3 fH between the pilot signals recorded in the mainly scanned track and the two neighboring tracks respectively and by comparing the levels of these two difference signals.
FIG. 2 shows in a block diagram the circuit arrangement of the VTR operating by the four-frequency method described above. Referring to FIG. 2, a reproduced signal consisting of a video signal which is reproduced by a rotary reproducing head and the pilot signals which are superimposed on the video signal comes from a terminal 5 to a low-pass filter (LPF) 6. The LPF 6 then separates the pilot signal component from the incoming reproduced signal. A multiplier 8 is arranged to perform a multiplying operation on the pilot signal component thus separated and a local pilot signal which is a reference signal generated by a local pilot signal generating circuit 7. The circuit 7 is arranged to produce a pilot signal or reference signal of the same frequency as that of the pilot signal recorded in the mainly scanned track. Then, as mentioned above with reference to FIG. 1, the output of the LPF 6 includes the frequency components f2, f4 and f3 with the track A2 assumed to be mainly scanned. In that event, the local pilot signal has the frequency f3. Therefore, the multiplier 8 produces a signal having frequencies representing the sum of and difference between the frequency f3 and the frequency components f2, f4 and f3. A band-pass filter (BPF) 9 is arranged to take out only a signal of frequency fH from the sum and difference signal while another BPF 10 is arranged to take out a signal of frequency 3 fH. The outputs of these BPFs are supplied to detection circuits 11 and 12 for detection and rectification.
The signal components fH and 3 fH which are thus obtained are then supplied to a level comparison circuit 13. The circuit 13 then produces a signal representing a level difference between these signal components. More specifically, when the reproduced level of the signal fH is higher than that of the signal 3 fH, a positive potential corresponding to the level difference is obtained. A negative potential is obtained in the opposite case. By this, a signal including information on the track deviating degree and the track deviating direction of the head is produced and can be used as a tracking error signal.
Under this condition, the relation between the deviating direction and the tracking error signal obtained for the track Ai becomes reverse to the relation obtained for the track Bi as mentioned in the foregoing with reference to FIG. 1. To solve this problem, therefore, a switching circuit 16 is arranged behind the level comparison circuit 13 to have the output put of the comparison circuit 13 selectively produced either through an inverting amplifier 14 or not through the amplifier 14 in accordance with a head switch-over signal 15.
In the conventional method of accomplishing tracking control by means of the pilot signals of four different frequencies as described above, the relation of the tracking error signal to the track deviating position of the head becomes as shown in FIG. 3. FIG. 3 shows the position of the head on the tape in the direction perpendicular to the recording tracks in relation to the level of the tracking error signal. In FIG. 3, a point T0 represents the position of the head when the head is normally tracing a track under control. Points T1, T2, T3 and T4 represent the head positions deviating respectively by one track pitch one after another. The axis of ordinate indicates the output of a terminal 17 (of FIG. 2) obtained at each of the points T0 to T4, that is, the level of the tracking error signal.
Assuming that the track pitch is approximately equal to the width of the head, the output of the terminal 17 which is the tracking error signal is zero at the point T0. This indicates that the tracking control is correctly performed on the reproducing head. However, if the tracking position of the reproducing head comes to deviate in the positive direction, the degree of tracking deviation increases showing a positive inclination until it reaches one track pitch, i.e. the point T1 representing the neighboring track. The deviating degree reaches a maximum value at the point T1. After that, the tracking error signal which is produced from the terminal 17 decreases showing a negative inclination according as the deviating degree further increases within the range of a two track-pitch distance (or between the points T1 and T3. During this process of level decrease of the tracking error signal, the level of the signal becomes zero at the point T2 which indicates deviation by a two track-pitch distance. After this point, the tracking error signal comes to be of a negative level. It reaches a minimum negative value at the point T3 which indicates deviation by a three track-pitch distance. When the tracking deviation degree comes to exceed the three track-pitch distance, the level of the tracking error signal again begins to increase showing a positive inclination and becomes zero at the point T4. In this manner, the level of the tracking error signal makes one cycle of changes while the tracking deviation increases from the point T0 to the point T4. This cycle is repeated according as the tracking deviation further increases.
While the tracking error signal is changing in this manner, the tracking control system is arranged to control the position of the reproducing head and that of the recording medium relative to each other to lessen the tracking position deviation when the level of the tracking error signal comes to increase in the positive direction at the point T0. It also controls the relative positions of the head and the medium to lessen the deviation when the error signal comes to decrease in the negative direction. Under this control, the reproducing head is eventually locked in position at the point T0 where the level of the tracking error signal becomes zero. Meanwhile, the level of the tracking error signal becomes zero also at another phase point T2. Then, if the level of the tracking error signal increases in the positive direction or decreases in the negative direction from the point T2, the relative positions of the reproducing head and the recording medium would be controlled in such a way as to enlarge the deviating degree. Therefore, this point T2 never becomes a stabilizing point.
In the conventional apparatus of the kind described, there are two points at which the level of the tracking error signal becomes zero within every four track-pitch distance. Although the reproducing head accurately traces the track under control at one of the two points, the head deviates to the extent of a two track-pitch distance at the other point, which never can be a stabilizing point. However, since the tracking error signal also comes to the zero level also at the latter point, it takes an excessively long time in bringing the head back to the former point which is a stabilizing point.